Constant torque control system for a variable displacement pump or pumps

ABSTRACT

A control system for a variable displacement pump or pumps driven by a vehicular engine for supplying pressurized fluid to implement actuators via implement control valves. Included is a servomechanism comprising a servoactuator section coupled to each variable displacement pump for varying the per cycle displacement thereof, a servovalve section for operating the servoactuator section by fluid pressure from a fixed displacement pump, and a control section for actuating the servovalve section. The servomechanism control section has a control piston displaced by variable fluid pressures from the variable displacement pumps against the effect of at least two springs. The springs are so arranged that their spring constant changes as the control piston travels to a predetermined position intermediate the opposite extreme positions thereof, in order to make possible the constant torque control of each variable displacement pump for the effective use of the engine output horsepower.

BACKGROUND OF THE INVENTION

This invention relates to a fluid operated control system for a variabledisplacement pump or pumps driven by a prime mover such as an internalcombustion engine. More particularly, the invention is directed to asystem for controlling the per cycle displacement of a pump or pumpssupplying fluid under pressure to implement actuators, as in a materialshandling vehicle such as an excavator, in accordance with the load beingimposed on the pump or pumps.

The use of variable displacement pumps is common in earth movers or likevehicles for powering various implement actuators on such vehicles. Aconstant torque control of the variable displacement pumps makespossible the most effective use of the output horsepower of the enginedriving the pumps. Typical of known methods for the constant torquecontrol of the pumps is the one wherein the pump displacement iscontrolled by the resultant of the delivery pressure of the pumps andthe force of a spring or springs opposing the pump pressure.

The prior art devices constructed in accordance with the above constanttorque control method have had in common the weakness that the springmeans for providing the force required for controlling pump displacementare too bulky for the provision of compact pump control devices desiredtoday. Such bulky spring means give rise to additional drawbacks such asgreat hysteresis and fluctuations in control performance. Pumps havebeen growing smaller and smaller in recent years by virtue ofadvancements in materials and design engineering. However, as the pumpcontrol devices have not been reduced in size because of the bulkyspring means, neither have been the pump assemblies incorporating suchdevices.

SUMMARY OF THE INVENTION

The present invention defeats the above noted weaknesses of the priorart and makes it possible to effect constant torque control of avariable displacement pump or pumps with a simple, lightweight, andcompact apparatus. Particularly when adapted for controlling two or morepumps, the invention makes it possible to control the displacement ofeach pump in accordance with the sum of the delivery pressures of thepumps for the most efficient use of the output torque of the prime moverdriving the pumps.

Basically, the invention provides a constant torque control system forat least one variable displacement pump operating under a variable load,having a fixed displacement pump supplying fluid under pressure to aservomechanism for varying the displacement of the variable displacementpump. The servomechanism broadly comprises a servoactuator section foracting directly on the variable displacement pump for varying thedisplacement thereof, a servovalve section for controlling fluidpressure communication between the fixed displacement pump and theservoactuator section, and a control section for controllably actuatingthe servovalve section in response to the variable output fluid pressureof the variable displacement pump.

The servoactuator section of the servomechanism has a servopistonslidably mounted in a servomechanism housing so as to define a pair ofopposed fluid chambers and operatively coupled to the varyabledisplacement pump for varying its displacement in response to fluidpressure suppled to the fluid chambers. Resilient means act on theservopiston to cause the same to normally hold the variable displacementpump at a maximum displacement. The servovalve section has a servovalvespool slidably mounted in the servomechanism housing for selectivelyplacing the pair of fluid chambers of the servoactuator section in andout of communication with the fixed displacement pump and with a fluiddrain. The control section comprises control piston means alsoreciprocably mounted in the servomechanism housing for displacement in afirst direction in response to the variable output fluid pressure of thevariable displacement pump, and multiple spring means for biasing thecontrol piston means in a second direction, opposite to the firstdirection, and normally holding the control piston means in an extremeposition in the second direction. The multiple spring means has at leasttwo springs adapted to vary the total spring constant thereof when thecontrol piston means passes a predetermined position intermediate theopposite extreme positions thereof in the first and second directions.

Also included in the servomechanism is a control lever having a pair ofopposite ends operatively engaged with the servopiston of theservoactuator section and with the servovalve spool of the servovalvesection and medially pivoted to the control piston means of the controlsection. The control lever functions to move the servovalve spool inresponse to the movement of the control piston means, thereby causingdisplacement of the servopiston in response to fluid pressure that isfed from the fixed displacement pump via the servovalve section, andfurther to feed back the displacement of the servopiston to theservovalve spool, thereby controlling the displacement of the variabledisplacement pump in accordance with the variable fluid pressure outputthereof.

The multiple spring means take the form of two helical compressionsprings concentrically nested one within the other in one preferredembodiment of the invention. The outer spring constantly acts on thecontrol piston means to bias the same in the second direction regardlessof the position thereof. The inner spring acts on the control pistonmeans to bias the same in the second direction in coaction with theouter spring when the control piston means travels to and past thepredetermined position in the first direction. In another preferredembodiment, the multiple spring means comprises two helical compressionsprings arranged collinearly with each other, with a movable spring seatinterposed therebetween for movement into and out of abutment against astop. The two collinear springs conjointly resist the travel of thecontrol piston means in the first direction until the control pistonmeans reaches the predetermined position, where the movable spring seatbutts on the stop. When the control piston means travels past thepredetermined position in the first direction, only one of the springsacts to resist such travel of the control piston means.

The multiple spring means of the above configurations make it possibleto control the displacement of the variable displacement pump with adesired constant torque curve, with the size of the servomechanismreduced to a minimum.

For simultaneously controlling two or more variable displacement pumps,the servomechanism of the above construction may be provided for eachpump. Each servomechanism responds to the resultant of the output fluidpressures of all the variable displacement pumps to control thedisplacement of one associated pump for the utmost use of the engineoutput.

The above and other features and advantages of this invention and themanner of realizing them will become more apparent, and the inventionitself will best be understood, from a study of the followingdescription and appended claims, with reference had to the attacheddrawings showing the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the general organization ofthe constant torque control system embodying the principles of thepresent invention, the control system being herein shown adapted forcontrolling two variable displacement pumps driven by a common internalcombustion engine;

FIG. 2 is a detailed sectional representation of one of the twoservomechanisms used in the control system of FIG. 1, shown togetherwith the two variable displacement pumps and one fixed displacementpump, the other servomechanism being of like construction;

FIG. 3 is a graph plotting the curve of the output fluid pressure ofeach variable displacement pump against its rate of delivery, ascontrolled by the control system of FIG. 1; and

FIG. 4 is a view similar to FIG. 2 but showing a modified servomechanismfor use in the control system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The general organization of the constant torque control system inaccordance with this invention will be best understood from aconsideration of FIG. 1. This figure shows the control system as adaptedfor use in an earth mover having two variable displacement pumps 10 and10' driven by a prime mover such as an internal combustion engine 12 forsupplying fluid pressure to two groups of implement actuators, notshown, via respective groups of implement control valves 14 and 14'.Both pumps 10 and 10' can be of conventional design having, for example,swash plates 16 and 16'. A change in the angular position of these swashplates results in a change in the per cycle displacement of the pumps 10and 10'.

The first variable displacement pump 10 draws fluid, preferablyhydraulic oil, from a reservoir or sump 18 and supplies it underpressure to the first group of implement control valves 14 by way of asupply conduit 20. Each in the form of a three position, directionalcontrol valve, the implement control valves 14 function under thevehicle operator's control to selectively place the pump 10 incommunication with the associated implement actuators and with a fluiddrain indicated at 22. The second variable displacement pump 10' islikewise connected to the second group of similar implement controlvalves 14' by way of a supply conduit 20'. The implement control valves14' also operate under the vehicle operator's control to selectivelyplace the second pump 10' in communication with the associated implementactuators and with the fluid drain 22.

For controlling the displacement of the pumps 10 and 10' there areprovided servomechanisms 24 and 24' of like construction which operatethe swash plates 16 and 16' of the pumps. Each servomechanism respondsto fluid pressures from both pumps 10 and 10'. Thus the first pump 10communicates with both servomechanisms 24 and 24' by way of a conduit 26and branch conduits 28 and 30, whereas the second pump 10' communicateswith both servomechanisms by way of a conduit 26' and branch conduits28' and 30'.

Also in communication with both servomechanisms 24 and 24' is a fixeddisplacement pump 32 of smaller capacity than the variable displacementpumps 10 and 10'. Driven by the engine 12, the fixed displacement pump32 delivers fluid under pressure to both servomechanisms 24 and 24' byway of a conduit 34 and branch conduits 36 and 36'. The conduit 34 has arelief valve 38 for bleeding off excess pressure from the fixeddisplacement pump 32. The fluid pressure thus delivered from fixeddisplacement pump 32 to servomechanisms 24 and 24' is to be used for thecontrolled actuation of the swash plates 16 and 16' of the variabledisplacement pumps 10 and 10', as will become more clearly understoodfrom the following description of the servomechanisms.

Given hereafter, with reference directed primarily to FIG. 2, is a moreextensive discussion of the above constant torque control system, andparticularly of the servomechanisms 24 and 24', the major components ofthe control system. Since the two variable displacement pumps 10 and 10'are controlled in a like manner, and since the two servomechanisms 24and 24' are of like construction, FIG. 2 shows only one servomechanism24 in its working relationship with the pumps 10, 10' and 32. Only theservomechanism 24 will therefore be described in detail, it beingunderstood that the same description applies to the other servomechanism24'.

The representative servomechanism 24 broadly comprises:

1. A servoactuator section 40 for changing the displacement of thevariable displacement pump 10 by acting directly on its swash plate 16.

2. A servovalve section 42 for controlling fluid pressure communicationbetween fixed displacement pump 32 and servoactuator section 40.

3. A control section 44 for actuating the servovalve section 42 inresponse to fluid pressures from both variable displacement pumps 10 and10'.

Perhaps extraneous to all these sections 40, 42 and 44 but still forminga part of the servomechanism 24 is a control lever 46 which serves tointerrelate the operations of those sections. An additional constituentof the servomechanism 24 is a housing 48 which is common to all itssections 40, 42 and 44.

The servoactuator section 40 of the servomechanism 24 has a servopiston50 slidably fitted in a bore 52 in the housing 48. Closing the oppositeends of the bore 52, a pair of end covers 54 and 56 coact with theservopiston 50 to define a pair of fluid chambers 58 and 60 on theopposite sides of the servopiston. The servopiston 50 has an actuatorarm 62 embedded therein and projecting out of the servomechanism housing48 through a slot 64 defined therein. The projecting end of the actuatorarm 62 is operatively coupled to the swash plate 16 of the variabledisplacement pump 10, in such a manner that the linear travel of theservopiston 50 within the bore 52 results in a change in the per cycledisplacement of the pump 10. The linear travel of the servopiston 50takes place as the opposed fluid chambers 58 and 60 are selectivelyplaced in and out of communication with the fixed displacement pump 32by the servovalve section 42. A helical compression spring 66 actsbetween servopiston 50 and left hand end cover 54 to bias the formertoward the extreme right hand position in which the servopiston isshown. Normally retained in this extreme right hand position underspring pressure, the servopiston 50 holds the variable displacement pump10 at maximum displacement.

For the selective delivery of the pressurized fluid from the fixeddisplacement pump 32 to the pair of opposed fluid chambers 58 and 60 ofthe servoactuator section 40, the servovalve section 42 of theservomechanism 24 has a spool 68 slidably received in a guide sleeve 70.This guide sleeve is closely fitted in a bore 72 in the servomechanismhousing 48, with the bore 72 extending parallel to the bore 52 of theservoactuator section 40. The left hand end of the guide sleeve 70 isheld against an adjusting screw 74 via a slidable abutment 76 having anO ring seal 78. The adjusting screw 74 extends through, and isthreadedly engaged with, an end cover 80 closing the left hand end ofthe bore 72 and has a locknut 82 fitted thereon. The right hand end ofthe guide sleeve 70, on the other hand, butts on a slidable spring seat84, which in turn is held against an adjusting screw 86 via a slidableabutment 88 having an O ring seal 90. The adjusting screw 86 likewiseextends through an end cover 92 closing the right hand end of the bore72 and has a locknut 94 engaged thereon. Mounted between servovalvespool 68 and spring seat 84, a helical compression spring 96 is intendedto prevent looseness among control lever 46, servopiston 50, andservovalve spool 68 which are interengaged in a manner yet to bedescribed.

The guide sleeve 70 of the servovalve section 42 has an inlet port 98and two outlet ports 100 and 102 formed radially therein. The inlet port98 communicates with the fixed displacement pump 32 by way of theconduits 34 and 36. The outlet ports 100 and 102 communicate with theopposed fluid chambers 58 and 60 of the servoactuator section 40 by wayof passageways 104 and 106, respectively, in the servomechanism housing48. The servovalve spool 68 functions to selectively place the inletport 98 in and out of communication with the outlet ports 100 and 102.The servovalve spool 68 has formed therein an axial bore 108 to providea drain passageway to be selectively placed in and out of communicationwith the outlet ports 100 and 102.

The servovalve spool 68 is further recessed medially at 110. Pivotablyand slidably engaged in this recess 110 is one end 112 of the controllever 46 extending with clearance through a recess 114 in theservomechanism housing 48 so as to extend between the bores 52 and 72therein. The other end 116 of the control lever 46 in similarly engagedin a recess or socket 118 in the servopiston 50 of the servoactuatorsection 40. The control lever 46 activates the servovalve spool 68 bybeing itself actuated by means in the control section 44, as well as bythe servopiston 50 of the servoactuator section 40, in a manner to bedetailed subsequently.

Normally, the servovalve spool 68 lies in the illustrated position,holding the inlet passageway 98 out of communication with the outletpassageway 102. The servopiston 50 of the servoactuator section 40 istherefore held in the illustrated maximum displacement position underthe bias of the compression spring 66. On actuation, then, theservovalve spool 68 selectively communicates the inlet passageway 98with the outlet passageways 100 and 102. The servopiston 50 will thentravel in a desired direction under fluid pressure from the fixeddisplacement pump 32.

Intended to actuate the servovalve spool 68 via the control lever 46,the control section 44 has a control piston 120 slidably received in abore 122 defined in the servomechanism housing 48 so as to extendparallel to the bores 52 and 72. The control lever 46 is mediallycoupled to the control piston 120 via a pivot pin 124. The controlpiston 120 has its right hand end held against a first biasing piston126 and thence against a second biasing piston 128, which biasingpistons are in axial alignment with the control piston 120 and slidablyreceived in respective guide sleeves 130 and 132. The second biasingpiston 128 normally butts on an end cover 134 closing the right hand endof the bore 122. The first biasing piston 126 has an annular shoulder136 to bear the variable fluid pressure fed from the first variabledisplacement pump 10 via the conduits 26 and 28 and a passageway 138 inthe servomechanism housing 48. The second biasing piston 128 also has anannular shoulder 140 to bear the variable fluid pressure fed from thesecond variable displacement pump 10' via the conduits 26' and 28' and apassageway 142 in the servomechanism housing 48. At 144 is seen a drainpassageway associated with the biasing pistons 126 and 128.

Arranged next to the left hand end of the control piston 120 aremultiple spring means 146 constituting a feature of this invention. Inthis particular embodiment, the multiple spring means 146 has a first148 and a second 150 helical compression springs of coaxial arrangementwith the control piston 120, with the second compression spring 150concentrically nested within the first compression spring 148. Bothfirst 148 and second 150 compression springs have their right hand endsheld against a common spring seat 152 on the left hand end of thecontrol piston 120. The first compression spring 148 has its left handend seated against an end cover 154 closing the left hand end of thebore 122 by being screwed or otherwise fastened to the servomechanismhousing 48. Thus the first compression spring 148 normally holds thecontrol piston 120 in its extreme right hand position, as shown, withthe second biasing piston 128 butting against the right hand end cover134. As required, a shim pack or the like may be installed between themating surfaces of servomechanism housing 48 and end cover 154 for theadjustment of the force of the first compression spring 148 resistingthe leftward travel of the control piston 120.

While the first compression spring 148 constantly acts to bias thecontrol piston 120 rightwardly, the second compression spring 150 startsresisting the leftward travel of the control piston 120 only when thelatter travels leftwardly to a predetermined position intermediate theopposite extreme positions thereof. Accordingly, the total springconstant of the two compression springs 148 and 150 changes when thecontrol piston 120 reaches the predetermined intermediate position.considerably less in free axial length than the first compression spring148, the second compression spring 150 is arranged for movement into andout of abutment against a spring seat 156. A guide core 158 projectsrightwardly from the spring seat 156 and is loosely received in thesecond compression spring 150 for guiding the movement thereof into andout of abutment against the spring seat 156. This spring seat 156 isitself adjustably movable toward and away from the control piston 120 byan adjusting screw 160 threadedly extending through and projectingoutwardly of the end cover 154, for correspondingly varying thepredetermined intermediate position of the control piston 120 where thetotal spring constant of the compression springs 148 and 150 changes. Alocknut 162 is fitted over the projecting end of the adjusting screw160.

Operation

The operation of the constant torque control system of FIGS. 1 and 2will be better understood by referring also to FIG. 3 graphicallyrepresenting the pressure-displacement characteristic of each variabledisplacement pump 10 or 10' under the control of that system.

Normally, or when all the implement control valves 14 and 14' are inneutral, with no load on the variable displacement pumps 10 and 10', thefluid pressure from these pumps does not cause displacement of thecontrol piston 120 of the control section 44 of the servomechanism 24(or of the other servomechanism 24') against the force of thecompression spring 148 of the multiple spring means 146. The controlpiston 120, as well as the biasing pistons 126 and 128, is in theillustrated extreme right hand position, holding the spool 68 of theservovalve section 42 also in the illustrated extreme right handposition. The pressurized fluid from the fixed displacement pump 32 isthen directed into the left hand fluid chamber 58 of the servoactuatorsection 40 through the conduits 34 and 36, the inlet port 98 and outletport 100 of the servovalve section, and the passageway 104 in theservomechanism housing 48. The right hand fluid chamber 60 of theservoactuator section 40 communicates with the fluid drain by way of thepassageway 106 in the servomechanism housing 48 and the port 102 anddrain passageway 108 of the servovalve section 42. With the servopiston50 of the servoactuator section 40 thus held in the illustrated extremeright hand position, the variable displacement pump 10 is at a maximumdisplacement Qmax.

When any one or more of the implement control valves 14 and 14, ineither or both of the two groups of such valves, are hand actuated tooperate the corresponding one or ones of the unshown implementactuators, the output pressure of one or both of the variabledisplacement pumps 10 and 10' will rise, exerting a leftward force oneither or both of the biasing pistons 126 and 128 of the control section44 of the servomechanism 24 in opposition to the force of the firstcompression spring 148 of the multiple spring means 146.

Let Pa1 be the output pressure of the first variable displacement pump10, and Pa2 the output pressure of the second variable displacement pump10'. Then, until the mean output pressure (Pa1+Pa2)/2 of the pumps 10and 10' builds up to [(Pa1+Pa2)/2]_(L), the control piston 120 of theservomechanism control section 44 will travel to the left against thebias of the first compression spring 148 of the multiple spring means146 under the fluid pressure acting on either or both of the biasingpistons 126 and 128. The control lever 46 will transmit such leftwardtravel of the control piston 120 to the spool 68 of the servovalvesection 42. The consequent leftward travel of the servovalve spool 68will result in a reduction in the cross sectional area of the fluidpassageway from inlet port 98 to outlet port 100. The servovalve spool68 will block the communication of the inlet port 98 with both outletports 100 and 102 when the mean output pressure of the pumps 10 and 10'builds up to [(Pa1+ Pa2)/2]_(L).

With a continued increase in the output pressure of the pump 10 and/orpump 10', the control piston 120 will further travel leftwardly againstthe force of the compression spring 148, resulting in the simultaneousleftward travel of the servovalve spool 68. Then the servovalve spool 68will place the inlet port 98 in communication with the outlet port 102and will place the other outlet port 100 in communication with the drainpassageway 108. Thus the servovalve section 42 will direct thepressurized fluid from the fixed displacement pump 32 toward the righthand fluid chamber 60 of the servoactuator section 40 by way of thepassageway 106 and communicate the left hand fluid chamber 58 with thefluid drain by way of the passageway 104. The result will be theleftward travel of the servopiston 50 against the force of thecompression spring 66, causing a decrease in the displacement of thevariable displacement pump 10.

Upon leftward travel of the servopiston 50, the control lever 46 will bepivoted in a clockwise direction about its pivot pin 124 on the controlpiston 120 and will so cause rightward displacement of the servovalvespool 68, thereby feeding back the displacement of the servopiston 50 tothe servovalve spool 68. Therefore, when the servopiston 50 reaches aposition corresponding to the total output pressure of the variabledisplacement pumps 10 and 10', the servovalve spool 68 will place theinlet port 98 out of communication with both outlet ports 100 and 102.The variable displacement pump 10 will thus be held at a displacementcorresponding to the total output pressure of the variable displacementpumps 10 and 10' at that time.

Until the mean output pressure of the variable displacement pumps 10 and10' builds up to [(Pa1+Pa2)/2]_(M) in the graph of FIG. 3, the controlpiston 120 of the control section 44 will travel leftwardly only againstthe force of the first compression spring 148 of the multiple springmeans 146, and the servomechanism will operate to control thediplacement of the variable displacement pump 10 to suit the totaloutput pressure of both variable displacement pumps 10 and 10'.

When the mean output pressure of the variable displacement pumps 10 and10' rises to [(Pa1+Pa2)/2]_(M), the control piston 120 will reach theaforesaid predetermined position intermediate the opposite extremepositions thereof. Then the distance between the spring seat 152 and 156will become equal to the free axial dimension of the second compressionspring 150 of the multiple spring means 146. Consequently, with afurther increase in the total output pressure of the variabledisplacement pumps 10 and 10', the control piston 120 will travelleftwardly against the combined forces of both compression springs 148and 150 of the multiple spring means 146, causing a simultaneousleftward travel of the servovalve spool 68. The servovalve section 42will then place the the right hand fluid chamber 60 of the servoactuatorsection 40 in communication with the fixed displacement pump 32, and theleft hand fluid chamber 58 in communication with the fluid drain. Theresulting leftward travel of the servopiston 50 will cause a decrease inthe displacement of the variable displacement pump 10. Also, with suchleftward travel of the servopiston 50, the control lever 46 will bepivoted in a clockwise direction to move the servovalve spool 68rightwardly against the force of the compression spring 96. Thus, whenthe servopiston 50 reaches a position corresponding to the total outputpressure of the variable displacement pumps 10 and 10', the servovalvesection 42 will close both fluid chambers 58 and 60 of the servoactuatorsection 40. The variable displacement pump 10 will then be held at adisplacement corresponding to the total output pressure of both variabledisplacement pumps 10 and 10'.

When the mean output pressure of the variable displacement pumps 10 and10' further rises to [(Pa1+Pa2)/2]_(H), the maximum, the servopiston 50will hit the left hand end cover 54 and will reduce the displacement ofthe variable displacement pump 10 to a minimum Qmin.

The operation of the servomechanism 24 when the total output pressure ofthe variable displacement pumps 10 and 10' decreases will be understoodfrom the foregoing description.

As is apparent from the above, only the first compression spring 148 ofthe multiple spring means 146 resists the leftward travel of the controlpiston 120 from point LP to point MP in the graph of FIG. 3. From pointMP to point HP, both compression springs 148 and 150 of the multiplespring means 146 combinedly resist the leftward travel of the controlpiston 120. Therefore, by suitably selecting the spring constants ofthese compression springs 148 and 150, the displacement of the variabledisplacement pump 10, can be controlled along the constant torque curve164 of FIG. 3 which is tangent to the two straight lines LP-MP andMP-HP. It will also be seen that the constant torque curve, as well asthe points LP, MP and HP, is variable by changing the springs of themultiple spring means 146. The intermediate point MP, in particular, isreadily variable by revolving the adjusting screw 160 into or out of theend cover 154.

As an additional advantage, the constant torque control system of thisinvention is applicable to two or more variable displacement pumpspositioned at a distance from each other. The pumps will be controlledin a hydraulically interlocked manner, to provide the samepressure-displacement characteristic.

SECOND FORM

In FIG. 4 is shown a modified servomechanism 24a for use in the constanttorque torque control system of FIG. 1 in substitution for eachservomechanism 24, 24'. The modified servomechanism 24a featuresmultiple spring means 146a, included in its control section 44, whichdiffer in both construction and operation from the multiple spring means146 of the servomechanism 24 or 24'.

The multiple spring means 146a comprises first 148a and second 150ahelical compression springs of collinear arrangement with respect toeach other and of coaxial arrangement with respect to the control piston120. The first compression spring 148a has one end held against thecontrol piston 120 via a spring seat 152a and another end held against afloating spring seat 166 movable relative to the servomechanism housing48 in the axial direction of the springs 148a and 150a. The secondcompression spring 150a has one end held against the floating springseat 166 and another end held against the end cover 154 closing the lefthand end of the bore 122 in the servomechanism housing 48. Normallyholding the control piston 120 in its extreme right hand position, thefirst spring 148a and second spring 150a coact to resist the leftwardtravel of the control piston under fluid pressure from the variabledisplacement pumps 10 and 10'.

Also included in the modified multiple spring means 146a is a stop orabutment 168 disposed within the second compression spring 150a. Thefloating spring seat 166 is to butt on the stop 168 when the controlpiston 120 travels leftwardly against the forces of the compressionsprings 148a and 150a to a predetermined position intermediate theopposite extreme positions thereof. The stop 168 has a guide core 170projecting therefrom toward the control piston 120 and loosely extendingthrough a hollow 172 in the floating spring seat 166 for guiding thelinear reciprocation thereof with the deflection of the compressionsprings 148a and 150a. Extending through and threadedly engaged with theend cover 154, the adjusting screw 160 is used in this embodiment foradjustably varying the position of the stop 168 toward and away from thecontrol piston 120.

The modified servomechanism 24a can be identical in the other details ofconstruction with the servomechanism 24 of FIG. 2. The constant torquecontrol system in which the modified servomechanism 24a is to beincorporated can also be configured as shown in FIG. 1.

Operation of Second Form

The operation of the constant torque control system of FIG. 4incorporating the modified servomechanism 24a can, of course, begenerally analogous with that of FIG. 2 except for the performance ofthe multiple spring means 146a.

The servomechanism 24a holds the variable displacement pump 10 at amaximum displacement in the absence of fluid pressure from the pumps 10and 10' to the biasing pistons 126 and 128 of the control section 44.The control piston 120 of the control section 44 will travel leftwardlyagainst the effect of the multiple spring means 146a upon loading of oneor both of the variable displacement pumps 10 and 10'. Until the meamoutput pressure from the variable displacement pumps 10 and 10' buildsup to [(Pa1+Pa2)/2]_(M), the control pistion 120 will travel against theresultant of the forces of both first 148a and second 150a compressionsprings of the multiple spring means 146a, with the floating spring seat166 moving toward the stop 168 with the compression of the springs 148aand 150a.

The floating spring seat 166 will hit the stop 168 when the controlpiston 120 travels leftwardly to the predetermined intermediate positionupon increase of the mean output pressure of the variable displacementpumps 10 and 10' to [(Pa1+Pa2)/2]_(M). Therefore, with a furtherincrease in the output pressures of the variable displacement pumps 10and 10', only the first compression spring 148a will be compressed toallow leftward travel of the control piston 120. The compression of thefirst compression spring 148a will continue until the mean outputpressure of the variable displacement pumps 10 and 10' rises to themaximum of [(Pa1+Pa2)/2]_(H), when the servopiston 50 of theservoactuator section 40 will strike the end cover 54 to reduce thedisplacement of the variable displacement pump 10 to a minimum. Thepredetermined intermediate position of the control piston 120, where itbutts on the stop 168 as above, is readily variable by turning theadjusting screw 160 into or out of the end cover 154.

The other details of the operation of the modified servomechanism 24aare considered self evident from the foregoing operational descriptionof the servomechanism 24 of FIG. 2, with reference directed also to thegraph of FIG. 3.

Thus the modified servomechanism 24a also makes it possible to controlthe displacement of the pump 10, or pump 10', along the constant torquecurve 164 of FIG. 3. In this embodiment, however, both first 148a andsecond 150a compression springs of the multiple spring means 146a resistthe displacement of the control piston 120 from point LP to point MP inFIG. 3, and only the first compression spring 148a resists thedisplacement of the control piston from point MP to point HP. Themultiple spring means 146a offers the additional advantage of a greateramount of deflection of the compression springs 148a and 150a madepossible by its collinear arrangement and, consequently, of the greateraccuracy with which pump displacement can be controlled.

Although the present invention has been shown and described as embodiedin the constant torque control system for two variable displacementpumps, it will be understood that the invention is readily adaptable foruse with one, three or more pumps. Further, while the multiple springmeans 146 and 146a each have but two springs in the illustratedembodiment, a greater number of springs could of course be employed forcontrolling the displacement of a pump or pumps along a constant torquecurve tangent to a correspondingly greater number of lines. These andadditional adaptations or modifications of the illustrated embodimentsmay be restored to without departing from the scope of the presentinvention.

What is claimed is:
 1. A constant torque control system for at least onevariable displacement pump operating under a variable load, said controlsystem consisting of a servomechanism for varying the displacement ofthe variable displacement pump and a fixed displacement pump supplyingfluid under pressure to said servomechanism, the servomechanismcomprising:(a) a housing; (b) a servoactuator section for actingdirectly on the variable displacement pump, the servoactuator sectioncomprising:(1) a servopiston slidably mounted in the housing so as todefine a pair of opposed fluid chambers and operatively coupled to thevariable displacement pump for varying the displacement thereof inresponse to fluid pressure supplied to the fluid chambers; and (2)resilient means acting on the servopiston for normally holding thevariable displacement pump at a maximum displacement; (c) a servovalvesection for controlling communication between the fixed displacementpump and the pair of fluid chambers of the servoactuator section, theservovalve section comprising:(1) a servovalve spool slidably mounted inthe housing for selectively placing the pair of fluid chambers of theservoactuator section in and out of communication with the fixeddisplacement pump and with a fluid drain; (d) a control sectionresponsive to the variable fluid pressure from the variable displacementpump for controllably actuating the servovalve section, the controlsection comprising:(1) control piston means slidably mounted in thehousing for displacement in a first direction in response to thevariable fluid pressure from the variable displacement pump; and (2)multiple spring means for biasing the control piston means in a seconddirection opposite to the first direction and normally holding thecontrol piston means in an extreme position in the second direction, themultiple spring means comprising at least two springs adapted to vary atotal spring constant thereof when the control piston means reaches apredetermined position intermediate the opposite extreme positionsthereof in the first and second directions, wherein said multiple springmeans comprises a first helical compression spring constantly acting onthe control piston means regardless of the position of the latterrelative to the housing to bias the control piston means in the seconddirection, a second helical compression spring arranged concentricallywith the first helical compression spring and having one end butting onthe control piston means, and a spring seat on which the other end ofthe second helical compression spring butts when the control pistonmeans travels to the predetermined position in the first directionagainst the bias of the first helical compression spring, the first andsecond helical compression springs cojointly resisting the travel of thecontrol piston means in the first direction past the predeterminedposition and means for adjustably varying the position of the springseat of the multiple spring means toward and away from the controlpiston means; and (e) a control lever having a pair of opposite endsoperatively engaging with the servopiston of the servoactuator sectionand with the servovalve spool of the servovalve section and mediallypivoted to the control piston means of the control section, the controllever being effective to move the servovalve spool in response to themovement of the control piston means in order to cause displacement ofthe servopiston in response to fluid pressure from the fixeddisplacement pump, and being further effective to feed back thedisplacement of the servopiston to the servovalve spool, therebycontrolling the displacement of the variable displacement pump inaccordance with the variable fluid pressure output thereof.
 2. Theconstant torque control system of claim 1 as adapted for controlling thedisplacements of a plurality of variable displacement pumps by providingone servomechanism for each variable displacement pump, the controlsection of each servomechanism being responsive to variable fluidpressures from all the variable displacement pumps.
 3. The constanttorque control system of claim 1 wherein the multiple spring means ofthe control section of the servomechanism comprises:(a) said firstspring constantly acting on the control piston means regardless of theposition of the latter relative to the housing to bias the controlpiston means in the second direction; and (b) said second spring actingon the control piston means to bias the same in the second direction incoaction with the first spring when the control piston means travels toand past the predetermined position in the first direction against theforce of the first spring.
 4. The constant torque control system ofclaim 3 wherein the first and second springs of the multiple springmeans are concentrically nested one within the other.
 5. A constanttorque control system for at least one variable displacement pumpoperating under a variable load, said control system consisting of aservomechanism for varying the displacement of the variable displacementpump and a fixed displacement pump supplying fluid under pressure tosaid servomechanism, the servomechanism comprising:(a) a housing; (b) aservoactuator section for acting directly on the variable displacementpump, the servoactuator section comprising:(1) a servopiston slidablymounted in the housing so as to define a pair of opposed fluid chambersand operatively coupled to the variable displacement pump for varyingthe displacement thereof in response to fluid pressure supplied to thefluid chambers; and (2) resilient means acting on the servopiston fornormally holding the variable displacement pump at a maximumdisplacement; (c) a servovalve section for controlling communicationbetween the fixed displacement pump and the pair of fluid chambers ofthe servoactuator section, the servovalve section comprising:(1) aservovalve spool slidably mounted in the housing for selectively placingthe pair of fluid chambers of the servoactuator section in and out ofcommunication with the fixed displacement pump and with a fluid drain;(d) a control section responsive to the variable fluid pressure from thevariable displacement pump for controllably actuating the servovalvesection, the control section comprising:(1) control piston meansslidably mounted in the housing for displacement in a first direction inresponse to the variable fluid pressure from the variable displacementpump; and (2) multiple spring means for biasing the control piston meansin a second direction opposite to the first direction and normallyholding the control piston means in an extreme position in the seconddirection, the multilple spring means comprising at least two springsadapted to vary a total spring constant thereof when the control pistonmeans reaches a predetermined position intermediate the opposite extremepositions thereof in the first and second directions, wherein the twosprings of the multiple spring means cojointly resist the travel of thecontrol piston means in the first direction until the control pistonmeans reaches the predetermined position, and when the control pistonmeans travels past the predetermined position in the first direction,only one of the springs acts to resist such travel of the control pistonmeans; and (e) a control lever having a pair of opposite endsoperatively engaging with the servopiston of the servoactuator sectionand with the servovalve spool of the servovalve section and mediallypivoted to the control piston means of the control section, the controllever being effective to move the servovalve spool in response to themovement of the control piston means in order to cause displacement ofthe servopiston in response to fluid pressure from the fixeddisplacement pump, and being further effective to feed back thedisplacement of the servopiston to the servovalve spool, therebycontrolling the displacement of the variable displacement pump inaccordance with the variable fluid pressure output thereof.
 6. Aconstant torque control system for at least one variable displacementpump operating under a variable load, said control system consisting ofa servomechanism for varying the displacement of the variabledisplacement pump and a fixed displacement pump supplying fluid underpressure to said servomechanism, the servomechanism comprising:(a) ahousing; (b) a servoactuator section for acting directly on the variabledisplacement pump, the servoactuator section comprising:(1) aservopiston slidably mounted in the housing so as to define a pair ofopposed fluid chambers and operatively coupled to the variabledisplacement pump for varying the displacement thereof in response tofluid pressure supplied to the fluid chambers; and (2) resilient meansacting on the servopiston for normally holding the variable displacementpump at a maximum displacement; (c) a servovalve section for controllingcommunication between the fixed displacement pump and the pair of fluidchambers of the servoactuator section, the servovalve sectioncomprising:(1) a servovalve spool slidably mounted in the housing forselectively placing the pair of fluid chambers of the servoactuatorsection in and out of communication with the fixed displacement pump andwith a fluid drain; (d) a control section responsive to the variablefluid pressure from the variable displacement pump for controllablyactuating the servovalve section, the control section comprising:(1)control piston means slidably mounted in the housing for displacement ina first direction in response to the variable fluid pressure from thevariable displacement pump; and (2) multiple spring means for biasingthe control piston means in a second direction opposite to the firstdirection and normally holding the control piston means in an extremeposition in the second direction, the multiple spring means comprisingat least two springs adapted to vary a total spring constant thereofwhen the control piston means reaches a predetermined positionintermediate the opposite extreme positions thereof in the first andsecond directions, wherein the multiple spring means comprises afloating spring seat movable relative to the housing in the axialdirection of the control piston means, a first helical compressionspring having one end held against the control piston means and anotherend held against the floating spring seat, a second helical compressionspring arranged collinearly with the first helical compression springand having one end held against the floating spring seat and another endheld against a stationary part, and a stop mounted in a fixed relationto the housing, the floating spring seat moving in abutment against thestop when the control piston means travels in the first direction to thepredetermined position against the forces of the first and secondhelical compression springs, so that only the first helical compressionspring resists the travel of the control piston means in the firstdirection beyond the predetermined position; and (e) a control leverhaving a pair of opposite ends operatively engaging with the servopistonof the servoactuator section and with the servovalve spool of theservovalve section and medially pivoted to the control piston means ofthe control section, the control lever being effective to move theservovalve spool in response to the movement of the control piston meansin order to cause displacement of the servopiston in response to fluidpressure from the fixed displacement pump, and being further effectiveto feed back the displacement of the servopiston to the servovalvespool, thereby controlling the displacement of the variable displacementpump in accordance with the variable fluid pressure output thereof. 7.The constant torque control system of claim 6 further comprising meansfor adjustably varying the position of the stop of the multiple springmeans toward and away from the control piston means.
 8. The constanttorque control system of claim 6 further comprising a guide coreprojecting from the stop and extending through a hollow in the floatingspring seat for guiding the movement of the floating spring seat intoand out of abutment against the stop.